The field is combustion chambers for internal combustion engines. In particular, the field includes constructions for opposed-piston engines in which a combustion chamber is defined between crown bowls of pistons disposed in opposition in the bore of a ported cylinder.
Per FIG. 1, a prior art opposed-piston engine includes at least one cylinder 10 with a bore 12 and longitudinally-displaced intake and exhaust ports 14 and 16 machined or formed therein. In some aspects the construction of the cylinder 10 includes a cylinder liner (or sleeve) that defines the bore. Fuel injectors 17 are secured in injector ports (ports where injectors are positioned) that open through the side surface of the cylinder. Two pistons 20, 22 with closed end regions (“crowns”) 20c, 22c are disposed in the bore with end surfaces 20e, 22e of the crowns facing each other. For convenience, the piston 20 is denominated as the “intake” piston because of its proximity to the intake port 14. Similarly, the piston 22 is denominated as the “exhaust” piston because of its proximity to the exhaust port 16.
Operation of an opposed-piston engine with one or more ported cylinders (cylinders with one or more of intake and exhaust ports formed therein) such as the cylinder 10 is well understood. In this regard, in response to combustion the opposed pistons move away from respective top dead center (TDC) positions where they are at their innermost positions in the cylinder 10. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions where they are at their outermost positions in the cylinder. The pistons may move in phase so that the intake and exhaust ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times. In other configurations, timing offsets can be implemented by placing intake and exhaust ports at different distances from the longitudinal center of the cylinder.
In many opposed-piston constructions, a phase offset is introduced into the piston movements. As shown in FIG. 1, for example, the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BDC positions in a sequence in which the exhaust port 16 opens as the exhaust piston 22 moves through BDC while the intake port 14 is still closed so that combustion gasses start to flow out of the exhaust port 16. As the pistons continue moving away from each other, the intake piston 20 moves through BDC causing the intake port 14 to open while the exhaust port 16 is still open. A charge of pressurized air is forced into the cylinder 10 through the open intake port 14, driving exhaust gasses out of the cylinder through the exhaust port 16. As seen in FIG. 1, after further movement of the pistons, the exhaust port 16 closes before the intake port 14 while the intake piston 20 continues to move away from BDC. Typically, the charge of fresh air is swirled as it passes through ramped openings of the intake port 14. With reference to FIG. 1, the swirling motion (or simply, “swirl”) 30 is a generally helical movement of charge air that circulates around the cylinder's longitudinal axis and moves longitudinally through the bore of the cylinder 10. Per FIG. 2, as the pistons 20, 22 continue moving toward TDC, the intake port 14 is closed and the swirling charge air remaining in the cylinder is compressed between the end surfaces 20e, 22e of the crowns 20c and 22c. As the pistons near their respective TDC locations in the cylinder bore, fuel 40 is injected into the compressed charge air, between the end surfaces 20e, 22e of the pistons. In some aspects, the fuel is injected directly through the side of the cylinder, into the bore, via the injectors 17 (“direct side injection”). As injection continues, the swirling mixture of air and fuel is increasingly compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e as the pistons 20 and 22 move through their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations.
Turbulence is a desirable feature of charge air motion as fuel injection begins. Turbulence encourages the mixing of charge air with fuel for more complete combustion. The geometries of the intake port openings and the cylinder of an opposed-piston engine provide a very effective platform for generation of an appropriate swirling motion of the charge air that promotes both removal of exhaust gasses (scavenging) and charge air turbulence. However, charge air motion that is dominated by swirl can produce undesirable effects during combustion. For example, during combustion in a cylindrical combustion chamber defined between flat piston end surfaces, swirl pushes the flame toward the cylinder bore, causing heat loss to the (relatively) cooler cylinder wall. The higher velocity vectors of swirl occur near the cylinder wall, which provides the worst scenario for heat losses: high temperature gas with high velocity that transfers heat to the cylinder wall and lowers the thermal efficiency of the engine. Accordingly, in such opposed-piston engines, it is desirable to maintain charge air turbulence as injection starts while mitigating the undesirable effects produced by swirl.
In certain opposed-piston combustion chamber constructions, turbulence is produced by squish flow from the periphery of the combustion chamber in a radial direction of the cylinder toward the cylinder's axis. Squish flow is generated by movement of compressed air from a relatively high-pressure region at the peripheries of the piston end surfaces to a lower-pressure region in a bowl formed in at least one piston end surface. Squish flow promotes charge' air turbulence in the combustion chamber. For example, U.S. Pat. No. 6,170,443 discloses a cylinder with a pair of opposed pistons having complementary end surface constructions. A circular concave depression formed in one end surface is symmetrical with respect to the axis of its piston and rises to a plateau in its center. The periphery of the opposing end surface has a convex shape in the center of which a semi-toroidal (half donut-shaped) trench is formed. As the pistons approach TDC, they define a generally toroidally-shaped combustion chamber centered on the longitudinal axis of the cylinder. The combustion chamber is surrounded by a circumferential squish band defined between the concave and convex surface shapes. As the pistons approach TDC, the squish band generates an inwardly-directed squish flow into the toroidal trench and creates “a swirl of high intensity near top dead center.” See the '443 patent at column 19, lines 25-27. Fuel is injected into the toridal combustion chamber in a radial direction of the bore
Increasing the turbulence of charge air in the combustion chamber increases the effectiveness of air/fuel mixing. Domination of charge air motion by swirl or squish flow alone does achieve a certain level of turbulence. Nevertheless, it is desirable to create additional elements of charge air motion as injection commences in order to produce even more turbulence of the charge air, which promotes more uniform mixing than can be obtained with swirl and/or squish, and to mitigate the effects of swirl thereby to reduce the carriage of the heat of combustion to the cylinder wall; one such additional component is tumble. In this regard, tumble is a rotating movement of charge air that circulates in a direction that is transverse to the longitudinal axis of the cylinder. Preferably, the tumbling motion is a circulation of charge air that circulates around a diameter of the cylinder bore.
An exemplary combustion chamber construction for an opposed-piston engine that generates a tumbling component of charge air motion is described and illustrated in US 2011/0271932. The engine includes at least one cylinder with longitudinally-separated exhaust and intake ports, and a pair of pistons disposed in opposition for reciprocating in a bore of the cylinder. As the pistons move toward TDC, a combustion chamber having an elongated ellipsoidal shape is formed in the bore, between the end surfaces of the pistons. The shapes of the end surfaces generate squish flows of charge air having complementary directions that are skewed with respect to a major axis of the combustion chamber. Interaction of the end surface shapes with squish and swirl components of the charge air motion causes generation of one or more tumbling motions in the combustion chamber.
Preferably, the end surface structures of the opposed pistons are identical in shape such that each end surface has a circumferential area centered on the longitudinal axis of the piston, and a bowl within the circumferential area that defines a concave surface with a first portion curving inwardly from a plane containing the circumferential contact area toward the interior of the piston and a second portion curving outwardly from the interior of the piston from the plane containing the circumferential contact area. The pistons are rotationally oriented to place complementary curved surfaces of the bowls in opposition in order to maximize the squish surface areas of the squish zone. These features result in a combustion chamber having the general shape of an elongated ellipsoid.
It has become apparent that a combustion chamber construction for an opposed-piston engine which is specified only as having the general shape of an elongated ellipsoid does not take into account features and dimensions of that “general shape” that can be varied individually, or collectively, in order to increase engine design flexibility and enable achievement of specific goals and requirements for opposed-piston engine performance. Accordingly, there is a need in the field of opposed-piston engine configuration for a systematic volumetric model with which to define combustion chamber constructions by characterization of features and dimensional relationships of combustion chamber elements.